It is conventional to lay paving stones individually by hand. However, this requires great expenditure for personnel and time and thus high costs for the laying.
An attempt disclosed in German Offenlegungsschrift No. 1,534,331, is known to avoid these disadvantages by prefabricating a group of individual paving stones in the form of a section of the ground covering to be produced, for instance composed of six rows of elongated paving stones in a composite structure, which are offset with respect to each other by a fraction of their length. This group is grasped as a whole by applying compressive force at two opposed outer sides of the group, for example at the two outer sides of the outside rows of the paving stones in the composite structure. This group is then moved over the pavement already laid to the place of laying where it is deposited in such manner that the laying pattern formed by the paving stones of the group will continue the laying pattern already existing.
It is customary e.g., as disclosed in East German Pat. No. 35,372 and British Pat. No. 208,182, to grasp individual paving stones, slabs to be laid, curbstones, embankment units, and the like from two sides by clamping pressure, lift them and convey them to a place of laying where they are deposited. This can be done safely even if individual elements of this kind are aligned in a single row one behind the other and this row is grasped from both sides so that each individual stone is held at both sides. As compared to that the invention is primarily but not exclusively concerned with the laying of groups of individual paving stones of the type in which at least one stone or, preferably, a plurality of stones are gripped at one side only, or not directly at all but instead are clamped only indirectly from one or more sides by neighboring stones. A borderline case under these conditions is one where two adjacent paving stones are grasped in such manner that the compressive force acting at two opposed sides acts directly at one outer side only of each paving stone and that the two paving stones are clamped against each other at their inner sides which face each other. This borderline case may be extended so that instead of each of the two paving stones a series of such paving stones is provided so that direct engagement of two adjacent rows of paving stones occurs only at one side each and that these adjacent rows of paving stones support each other at their sides facing each other under the action of the outer clamping force only. The range of application of the invention thus begins in particular in the two cases mentioned last of a pair of paving stones or of two rows of adjacent paving stones. Difficulties are encountered, above all, if one paving stone, as in the above mentioned known example, or preferably a greater number of paving stones or even a plurality of rows of paving stones, as in the cited example of six adjacent rows of paving stones in a composite structure, are held by clamping pressure alone by being framed by the outer paving stones which are directly grasped at one side. This is the normal case where a section of the ground covering to be made is prefabricated from individual paving stones of a composite structure by means of a conventional composing machine. Typical dimensions of such a section are, for example, a width of 60 cm and a length of 120 cm or, more generally, a covering area of between 0.5 and 1.5 m.sup.2. The invention encompasses the laying of any size of paving stone on the market, including the composite stone disclosed in German Pat. No. 1,459,739.
In cases of the kind mentioned last, in which at least one of the paving stones is directly exposed to compression at best at one side or perhaps not at all, it proved that so far the group of individual paving stones arranged loosely beside one another cannot be grasped reliably when lying in one layer by applying compressive force from two opposed sides and cannot be hoisted and moved safely to the place of laying where they are to be deposited. Rather, the group grasped does not remain stable, but instead indirectly held stones fell down and caused the entire group to collapse. A great number of unreliable factors contribute to this situation, especially with concrete stones, among them irregularities of the surface structure, trapped sand or stone granules or other foreign matter, etc.
In a known method and the corresponding apparatus, disclosed in German Offenlegungsschrift No. 1,534,201, it was attempted to improve the known method mentioned, disclosed in German Offenlegungsschrift No. 1,534,331, by different measures so as to adapt it to actual practice. For instance, holding surfaces of gripping tongs serving to grasp the group of individual paving stones were provided with a cover of rubber-elastic material. As composite structure paving stones should be laid with projections and indentations along their periphery, the cover was given a surface structure with projections and indentations corresponding to those of the stones to be grasped directly. The gripping jaw was provided with a height stop so as to guarantee that the group of individual paving stones could be grasped only in their upper range, or height region, so that among others sufficiently large foot space of the group grasped would remain free to permit easy joining of the group to a composite arrangement of stones already laid. Above all, however, special measures were taken to avoid that the loose paving stones grasped would break down. To this end separate spacers or, if desired, integral spacers of adjacent paving stones were disposed between the individual paving stones or it was prescribed that the paving stones should be slightly widening upwardly so as to be able to form an arched structure when being grasped. However, it is irrealistic and, to say the least, involves undue extra expenditure to install separate spacers. On the other hand, mold removal causes problems if the spacers are to be formed integrally with the paving stones in their lower range in laid condition where they are practically out of sight, or if the paving stones are of such shape as to widen conically toward the top. In conventional paving stone machines the mold is retracted in upward direction. With the known shaping mentioned in accordance with the preamble, however, the stones produced subsequently would have to be rearranged by exchanging the upper side against the under side so as to arrange them in an order ready for laying. Apart from that, however, perfect retention still is not obtained when grasping a group of individual paving stones of dimensions in accordance with practical requirements. As far as the applicant knows, this causes the last apparatus for carrying out the known method to be scrapped about a year and a half ago because the known method in accordance with the preamble was considered to be without any chances.
Instead the development of methods for the mechanical laying of paving stones went in different directions in the last few years.
Instead of the lateral grasping of a group of individual paving stones by compressive force, as discussed, a known method, disclosed in German Published Application No. 1,534,193, provides to hold this group of individual paving stones by suction acting from above. However, this is not only very expensive but also susceptible to trouble in operation because the surface structure, sand, and the like, at least of concrete paving stones, obstruct close contact between the paving stone and a suction head, to say the least. Moreover, it must be assured that no flow short circuit is caused by the joints between the individual paving stones as this might cause the suction action to break down.
Therefore, in the past few years it was attempted more and more to carry out the mechanical laying of paving stones by combining the paving stones from the very beginning in mechanically joined groups of elements. To achieve that, either individually made paving stones can be combined in a mechanical laying unit by means of a separate cement substance, as disclosed in German Offenlegungsschrift No. 1,459,669, or concrete bridges designed as rated or optional breaking zones can be provided between the individual paving stones, as also disclosed in Offenlegungsschrift No. 1,459,669. Thus in practice a large plate is formed consisting of partial elements which either only give the optical impression of individual paving stones or are broken up again subsequently into individual paving stones by ground vibrators. This, however, requires at least extra expenditure as compared to the laying of individual paving stones.
Further advantages of laying loose composite structure paving stones are to be seen in the fact that damaged stones can be exchanged easily, that it is simple to lay colored stones in pavements, such as for marking parking symbols or delimiting lines, that pavement stones can be removed readily for repair work, also underground work such as the laying of telephone cables, and that finally the composite structure laid disposes of homogeneous flexibility. This is in contrast to laying units not completely decomposed into individual parts with which, upon application of a localized, or paint-like, load, plate-like ground covering areas could be stressed by lever action, lifting adjacent elements.